Telephone system



April 4, 1939 laA A. HENSLER 2,153,306

TELEPHONE SYSTEM Filed Aug. 17, 1935 l2 Shee'ts-Sheet l WS klei-GH".

April 4, 1939. laA A. HENSLER 2,153,306

TELEPHONE SYSTEM Filed Aug. 17, 1955 l2 Sheets-Sheet 2 fm2' .4. 4 65 I+ 50 /1/0` P0 5K5 V L@ p4 i 5 W52 j/ N TU |073 HI' 2 -f April 4, 1939- l B. A. HENSLER 2,153,306

' TELEPHONE SYSTEM April 4, 1939- 'B A. HENSLER 2,153,306

TELEPHONE SYSTEM Filed Aug. 17, 1955 12 Sheets-Sheet 4 mig-2*- lOfl/ plil4, 1939- v B. A HENSLER 2,53,306

TELEPHONE SYSTEM Filed Aug. 17, 1955 l2 Sheets-Sheet 5 i /PL April 4, 1939. B. A. HENSLER 2,153,305

TELEPHONE SYSTEM Filed Aug. 17, 1935 l2 Sheets-Sheet 6 @W6 @ze /L I ax aye b n l' I: 06.3.- l '/04 af; 7

April 4, 1939. B. A. HENSLER 2,153,306

TELEPHONE SYSTEM Filed Aug. 17, 1955 l2 SlleetS--Sheefl 7 /v/v has To H3306 April 4, 1939- B. A. HENSLER TELEPHONE SYSTEM Filed Allg. 17, 1935 l2 Sheets-Sheet 8 Ffa. 4.1

April 4, 1939.

B A. HENSLER TELEPHONE SYSTEM Filed Aug. 17, 1935 l2 Sheets-Sheet 9 April 4, 1939- B A. HENSLER 2,153,306

TELEPHONE SYSTEM Filed Aug. 17, 1935 l2 SheeS-Sheet lO April 4, 1939- B A. HENSLER 2,153,306

TELEPHONE SYSTEM Filed Aug. 17, 1935 l2 Sheets-Sheet ll l' bz/ April 4, 1939- B A. HENsLx-:R 2,153,306

TELEPHONE SYSTEM Filed Aug. 17, 1935 l2 Sheets-Sheet l2 Patented Apr. 4, 1939 UNITED STATES PATENT OFFICE TELEPHONE SYSTEM Application August 17, 1935, Serial No. 36,703 In Great Britain August 22, 1934 14 Claims.

The present invention relates to telephone systems in which connections are set up over selecting switches which are set in accordance with markings applied to them.

In. this invention a selecting switch has associated with it for the purpose of setting it a device operable by coded signals transmitted over a signalling circuit from apparatus for setting up a connection automatically and the device according to a setting assumed in response to a coded signal makes marking connections to bring about the setting of the selecting switch to a particular position or group of positions. The device may conveniently take the form of relays less in number than the marking connections they are adapted tomake, the marking connections being made over serially connected contacts of the relays, and may be included in a control circuit available in common to a number of selecting switches.

In a multi-cnice area the selecting switches are commonly set under the control of a register sender in which the digits characteristic of the wanted number are stored and are retransmitted as impulse trains to the selecting switches in turn as they become connected up. In the present invention the registers, after being set transfer their settings in code form over signalling wires to the code responsive devices associated with the selecting switches as these become connected up. When the registers of a register sender receive their settings from an operators position they may be, and usually are, set by the transmission of the digits in code form from a key sender.

When a selecting switch extends the connection to a succeeding circuit the associated device is disconnected and a signal is transmitted to the register sender to bring about the application of the code signal for the marking of the succeeding selector. This method of setting selecting switches is convenient and rapid for switches in the same exchange as the register sender especially when the selecting switches are high speed motor driven switches having a hunting speed of 200 contacts or more a second, but since other exchanges may be involved in a connection the register sender is also adapted to transmit digits in impulse form. To bring about the change from code sending to impulse sending .a signal is transmitted to the sender from apparatus associated with a line outgoing from the exchange, this signal differing from that transmitted after a switch has switched through a connection. The signal for bringing about the application of a succeeding code marking may be a fleeting signal of the nature described in British Patent No. 422,200 while the signal for bringing about the change in the method of digit transmission may be of less iieeting nature which takes eifect on apparatus unresponsive to the eeting signal or it may be a signal transmitted by the application of a potential different from that normally applied during code reception o-r employed for the aforementioned purpose.

` For the connection of the registers a single switch may be employed which in its early positions connects up the registers in turn to receive the digits and later connects the registers to the signalling wires for the transfer of their settings in code form to the selecting switch code responsive device and for applying the requisite markings to the bank of a sending switch for impulse transmission.

The code responsive device for the marking of a selecting switch is conveniently situated in a common control circuit and may be connected to the signalling wires over back contacts of the selector switching relay, the change-over of these contacts on the operation of the switchingrelay serving to transmit the fleeting signal before referred to.

A selecting switch may be arranged to be set in accordance with two digits. In such a case a single motion switch may be included in the common control circuit and be marked by the operation of the code responsive device for the rst digit the switch hunting for the marked position and preparing a marking circuit for the selecting switch. The code responsive device is then released and the signal to the register sender transmitted by relay operationin the control circuit and the operation of the code responsive device by the second digit may effect a direct marking connection to the selecting switch bank, the testing circuit for the selecting switch including a wiper of the signal motion switch in the position to which it was set by the rst digit and the contacts of the code responsive device as determined by the second digit. The second digit may determine that a hunting operation by the selecting switch is required-or that the switch is to select an individually marked line.

In the case of trunk line connections the trunk selecting switches may be set in accordance with the invention, the connection being completed over a connecting circuit, which may be a cordless one at an operators position. To simplify the arrangements for connecting trunk lines and to lessen the work of the operators it may be arranged to connect repeaters into circuit if the trunk lines connected requi-re it. Further, the repeaters may, for simplicity be furnished with a small number of, :for example, two, fixed values of gain and the characteristics of the trunk lines with which connection is made may determine which of the values of gain is to be inserted. The repeaters may be in a group accessible toany connecting circuit on the trunk switchboard or commonv to an operators position or groupof positions and may be connected to a connecting circuit by means of a hunting switch. Relays in the connecting circuit operable from incoming and outgoing trunk line circuits may determine by their operation that a repeater is required and the value of gain to be inserted.

An embodiment of the invention vas applied to a trunk exchange is shown by way of example in the accompanying drawings. In the drawings Fig. 1 shows a both way trunk line circuit with a distributor for connecting incoming calls to a connecting circuit at an operators position or to a lamp display circuit; Fig. 2 shows a cordless connecting circuit with which a repeater may be associated automatically if required by the characteristics of the trunk lines which become connected to the connecting circuit, two values. of gain being provided for one of which is inserted automatically in accordance with the characteristics of the trunk lines connected; Fig. 3 shows a rst trunk selector, Fig. 4 a second trunk selector, Fig. 5 a reverting selector all set by markings applied by relays in response to a code and Fig. 6 a register sender.

All trunk calls are set up by operators by means of cordless connecting circuits and the selecting switches in the trunk exchange are high speed motor driven switches set in response to'a marking. This marking is applied by a set of 4 code Vrelays the relays being operated from the sender by a code, that is to say one or a combination of code relays is operated depending on the application of potential at the sender to one or a combination of 4 wires which extend between the sender and the selecting switches. The same wires serve for the transmission to the sender of a transient signal when the connection is switched through at a selecting switch to bring about the application of the code marking for the setting of the next selector. Provision is made in the sender for the transmission of from one to four digits in code form and the remainder, up to eight digits in all, in the usual impulse form.

The trunk selectors have the minimum of apparatus individual to them, the control being vested in common circuits. Provision is made for the storage of calls that cannot be extended by the second trunk selector due to congestion and for alternative routing in the case of some trunk groups. The second trunk selector is also enabled to search in a trunk group or to select an individuallinc in a group and to remain in contact with that line if it is found busy. The first trunk selector has access both to second trunk selectors and to trunk lines, no storage facilities being provided in the latter case. The reverting trunk selector becomes connected to the answering side of a connecting circuit by means: of a finder switch and gives access to second trunk selectors.

An operator normally has a connecting circuit assigned for use and which is seized by the distributor associated with a trunk line on the initiation of an incoming call, and the connection is set up by the operator by means of a key strip and the necessary keys and relays in a circuit common to all the connecting circuits at her position. This circuit is not shown in detail, only those keys and apparatus necessary for the understanding of theinvention being shown below the broken lin-e in Fig. 2. The position circuit follows the general lines of that shown and described in British Patent No. 401,394.

The operator setting up a connection has cornplete supervision of it while it is in being.

If a connecting circuit is not assigned an in coming call causes the distributor to test in to a free lamp in a lamp display on which all incoming calls for which connecting circuits are not assigned are queued up in order of their arrival. The calls are dealt with in order of arrival when a connecting circuit becomes assigned, the release of the call at the head of the queue and the stepping up of the others being performed automatically as described in the copending application of Bernard A. Hensler and Charles L. Peters, Serial No. 37,484, led Aug. 23, 1935.

The several gures forming the drawings vto the present specication and the circuits shown therein will now be considered in detail.

Consideration will rst be given to Fig. l. The circuit shown is connected to a trunk line incoming on the left hand side .of the drawing and has access over a distributor switch to connecting circuits, such as that shown in Fig. 2, the leads to which are shown outgoing on the right hand side of the drawing. If a connecting circuit is not assigned at the time the switch is hunting the switch makes connection with a call lamp display which may be of the kind shown in the copending Hensler and Peters application. Besides having access to connecting circuits at incoming positions the switch can gain access toa delay position to deal with calls which the operator receiving them has been unable to complete at the time and which have been ticketed and become reverting calls. A calling lamp and answering key such as would be provided on a delay position are shown in a broken line rectangle in this figure, the lamp and key being individual to the incoming trunk line. Access to the circuit for reverting calls is had over second trunk selectors such as that shown in Fig. 4 the leads from which enter the gure at the upper edge.

'I'he distributor switch is of the motor driven type in which the circuit of a small electric motor is controlled by a latch magnet designated LM, the magnet when operated disengaging a latch from the switch driving mechanism and connecting up the motor circuit. On de-energisation the latch magnet disconnects the motor circuit and reengages the latch with the driving mechanism. For purposes of description in this and other gures where motor driven switches are employed the energisation and de-energisation of the latch magnet will be assumed to include the starting and stopping of the motor. 'I'he latch magnet LM is under the control of a high speed relay T. The switch is of the 200 outlet type in which the contacts are connected in the equivalent of two sets of arcs of 104 contacts each, one set being accessible over back contacts of a wiper switching relay WS, and the other set over front contacts of this relay. An arc of contacts comprises in fact two arcs of 52 contacts each wiped over in succession by the twoy ends of a staggered wiper equivalent to one wiper moving over an arc of 104 contacts.

A call is initiated on receipt of ringing current over the trunk line, a suitable call responsive device being connected to the wire RRW. The response of this device to a call effects the connection of earth to wire STW the effect of which is to bring about the operation of relay ST over a circuit including contacts gs4, ssl, key NSK, and contact ss. Relay ST operates and locks up over contact stl. Contact st2 closes a circuit for the operation of a recording device to record the receipt of an incoming call over the trunk, contact st4 disconnects earth from a group control relay GP associated with a storage device for outward trunk calls which cannot be completed immediately while demand Working is in force. Relay GP is earthed over contacts in each trunk circuit of a group of trunks. Contact st5 opens the test wire PO incoming from second trunk selectors and contact stl closes a circuit for the magnet LM of the distributor switch. Magnet LM operates in a circuit from earth over contacts tl, sl, ds3, tf5, stl, magnet winding, relay RA tobattery and the switch is set in motion. As soon as the wipers leave their normal position the 01T- normal contacts on close. For the sake of description it will be assumed that a connecting circuit connected to the arcs associated with the wipers connected to the front contacts of relay WS has been assigned by an operator at an incoming position. The switch rotates until the last bank contacts are encountered whereupon a circuit is closed from earth over wiper 5, contacts wsll, ss4, dt2, rectier R, both windings of relay T, contacts (m5, ws5, wiper l, relay WS and its shunt to battery. Relay T operates in this circuit to stop the motor by opening contact tl which removes a short circuit from relay S which may operate in series with the magnet LM and locks up over its front contact sl, this contact being of the make-before-break type. Magnet LM will not operate in series with relay S owing tothe resistance of the latter. Shortly after the operation of relay T relay WS operates, disconnecting the odd numbered wipers and connecting up the even numbered wipers. In doing so it opens its own operating circuit and that of relay T but relay WS has locked up meanwhile over contact and, its early clasing contact wsl and contact on. Relay T however releases and short circuits relay S and causes the motor to restart in search of the assigned connecting circuit. The last contact in the dotted arc of wiper 4 is commoned with the last twelve contacts in the inner arc and earth over contacts ss3, s3, wiper 4 short circuits relays SS and DT and prevents their operation. C'onsequent on the switch advancing through its normal position contact on opens but owing to the high speed at which this contact is passed relay WS does not release. The switch continues in rotation and when the contacts connected to the assigned connecting circuit are reached relay T again operates the circuit being from earth over wire N, the application of which will be clear from the subsequent description of the circuits of Fig. 2, wiper contacts wsli, ss4, dt2, relay T, wiper 8a, wire P, to battery over shunted relay B in the connecting circuit. Relay S re-operates and relays DT and SS operate over contacts on and s5 and hold over contact ssl and relay ST is released by the prolonged opening of contact ssB. The speaking wires ar-e switched through over contacts ss2, s2, and ss3, s3 and the higher resistance upper winding of relay T after being short circuited by contact tl is disconnected at contact ss the relay remaining held over its lower resistance lower Winding. During the releasing period of relay ST battery is connected over resistance Rl which is of a value to denote the nature of the trunk line, contacts stt, s4, ssd,

'wsll, wiper 6, wire N to relay NA in the connecting circuit.

On the release of relay ST wire N is connected to earth over back contactst, contact gsl relay SY and contact e2. Relay SY is not operated at this time owing to the resistance in the circuit. Contact ssl opens the operating circuit for r-elay ST and connects relay Z to wire STW to enable it to respond to a re-ring should one be given. Contact dt3 opens in the circuit over wire PO and contact dtd in the group control circuit. Contact dt5 substantiates contact st2.

If the night service key NSK had been thrown, relay TF would have operated instead of ST and locked up over contacts tfl and stl. Contact tf2 closing, the lamp CL on a special transfer position is lit, and on seeing the lamp glowing the operator at that position presses the key ANK associated with the lamp CL and individual to the trunk to operate relay AN. Relay AN locks up over contacts rml and tfll, opens the circuit of lamp CL at contact cm2 and at contact cm3 closes a circuit for relay ST which thereupon operates,

releases relay TF', holds relay AN at contact st3 connecting position is unable to deal with a call she may, by applying battery potential to the negative wire, operate relay TF over its left hand winding to transfer the call to a transfer position. Relay TF locks up as previously described and lamp CL is lit. On the operation of relay AN consequent on the depression of key ANK by the transfer operator relay T is released followed by relay S. The switch now advances to its normal position magnet LM being energised over contacts on, s5, and stl, and when the normal position is reached the switch stops and relay SS is released. Relay ST can now operate over contacts ssl, tf3 and cm3 and the switch continues in Search of the assigned transfer operators connecting circuit.

If, when a call arrives no connecting circuit is assigned the distributor switch on being set in motion is advanced until a free lamp display circuit is found. This is marked by battery potential on the wires LD or if the lamp display be full, a battery potential will be found on wire LDO connected to the arc of wiper 8a. Assuming for the moment that a free lamp in the display is available then relays T and WS are operated when the switch reaches the last contacts in the arcs of the odd numbered wipers and relays and the switch is then restarted in the manner before described and continues in motion until the free display lamp is found. The circuit for relay T is then from earth over contacts ss5, ds2, arc and wiper 6, contacts ws4, ssd, dtZ, rectifier R, relay T, contacts an, ws5, wire LD to battery. Relay S reoperates but relay SS does not as it is short circuited over arc and wiper 4, contacts s3 and ss3. A circuit is now closed from earth over contacts ss2, s2, ws2, wiper 2, commoned contacts preceding the last Contact, upper winding of relay DS to battery. Relay DS operates and locks up over its lower winding and contacts dsl and ws2 and opens the operating circuit for relay T at contact ds2. Relay T however holds over its lower winding to earth over contact tl. Contact ds4 operates relay DT which locks up over contacts dtl and stl and contact ds3 prepares a circuit for magnet LM. Contact tl by connecting earth over the low resistance lower winding of relay T marks the particular lamp circuit busy to other testing distributors.

When an operator is free to take the calls, that is, after preceding calls, if any, indicated in the lamp display have been answered and the call in question is now at the head of the queue the circuit over wire LD is opened and relays T and S are released. 'I'he switch now advances until the assigned connecting circuit is reached, magnet LM being operated over contacts on, S5, ds3, tf5 and stl until the normal is reached whereupon the relay WS releases, releases relay DS and the switch is again started, magnet LM being now energised over contact tl.

When no free lamp display is available the switch rotates until the overflow outlet LDO is reached whereupon it is stopped, the overow outlets from the several switches being connected over individual resistances to a common battery terminal. Release from the overow outlet takes place when a connecting circuit is assigned.

In the event of a re-ring being received over the trunk line after relay SS has been operated relay Z is operated and locked up over contacts al, syl and s6. Contact a2 disconnects steady earth potential from wire N and connects this wire to an intermittent earth potential circuit thereby causing the operators supervisory lamp t llash; On seeing this the operator throws her speaking key, thereby reducing the resistance in the circuit to an amount which enables relay SY to operate. Relay SY operating contact syl releases'relay Z which in turn restores the steady earth potential on wire N.

After the conversation over the trunk line is completed the distributor is restored to normal by the opening of the circuit over wire P. Relays T, S and AN if operated, release and the switch rotates to normal whereupon relays SS and DT release.

When the trunk line is wanted for an outgoing call test is made for battery over the test wire PO incoming from the second trunk selector contacts m3, dt3, st and shunted relay GS. Relay GS operates and at contact gsl connects earth over relay SY to the wire SO. Contact gsd disconnects wire STW from relay ST and connects it to relay Z, contact gs5 operates the traiiic recorder for outgoing calls, contact gs2 closing causes lamp ORL to glow steadily and contact gs3 opens in the group control circuit.

To ring over the trunk line relay RR is operated by battery connected to the positive wire over a key in the connecting circuit. Contact 11'I closes to start up the ringing motor-generator if it is not already running and contacts 112 and w3 apply ringing current over wire CSW to the trunk line.

Whenever the switch motor is connected up relay RA operates in series with the latch magnet LM and the motor windings and at contact ral closes a circuit for a delayed alarm relay. Contact mi! connects lamp ORL to a wire which is earthed intermittently to cause the lamp to flash and contact 7a3 opens in the test wire PO to guard against interference by an attempted outgoing call.

The incoming connecting circuit shown in Fig.

2 is accessible over the both way trunk circuit of Fig. 1 and is also provided with means whereby by means of a reverting selector such as that shown in Fig. 5 access may be had over the answering side of the connecting circuit to the trunk circuit for thev purpose of calling backV over the trunk line for a call which has suffered delay and has been released.

A repeater with fixed values of gain is adapted to be connected automatically to the connecting circuit over a hunting switch, its insertion and that of one of two values of gain being dependent on the trunk lines with which connection is made by each side of the connecting circuit.

The connecting circuit is a cordless one and provides a straight through speaking connection with or without the insertion of a repeater after the call has been set up, supervisory relays being connected to the supervisory wires N and S, on either side of the circuit. The wires leading towards the bottom of the figure are connected to a position circuit common to all the connecting circuits at a position and which contains keys and relays for monitoring and supervisory and other purposes, a sending key strip and keys and relays for the starting of a register sender. The position circuit may follow the general lines of that shown in the British Patent previously referred to, only those parts necessary for the proper understanding of the present invention being shown or referred to. The connecting circuit itself follows the general lines of that described in the aforementioned specication suitably modified to enable the automatic switching in of a repeater to be accomplished.

When an operator is free to deal with an incoming call she throws her speaking key SP thereby operating relay SK. Contacts slcl and sk2 split the speaking wire circuit and connect the calling side to the operators circuit, and contacts sk3 and sk4 connect the answering side to the operators circuit. Contact slc closes a circuit for relay SL over contact b5 and a contact of the speaking key and relay MN in the position circuit, contacts sk and slc'l close points in circuits for relay B and contact Sk8 short circuits the upper winding and connects earth over the low resistance lower winding of relay SK to prevent the operation of a relay similar to SK in another connecting circuit and thus bringing about interference by the connection together of two connecting circuits. Relay SL operates and disconnects the high resistancey winding of relay SK at contacts sIB and holds relay SK over its low resistance winding independently of the speaking key. Contacts sZ2 and S13 close further points in circuits for relay B, contact S14 extends the outgoing wire P to the position circuit and contact SI5 prepares a circuit for relay SD. The connect answer key CA is now thrown.

If there are calls waiting and connected to .the lamp display circuit a contact of this key, not shown, in its operated position brings about the release of the call from the display by earthing the wire LD shown to the right of Fig. 1 thereby releasing relay T in that ligure. The method of applying the earth to release the call at the head of the queue is shown in the co-pending Hensler and Peters application. The call distributor in the trunk circuit now commences to hunt for the assigned connecting circuit which is marked by battery potential over shunted relay B, contact slcB, release answer key RA, connect answer key CA, contacts sl3, TS2, and wire P.

=If there had been no calls connected to the lamp display circuit earth would have been connected to wire EN over a contact corresponding to contact enl in Fig. 2 of the above mentioned copending application and an incoming call finding a connecting circuit already assigned the test relay oi the call distributor would be connected at one end to this earth over wire EN, contact b2 and wire N and the other end to battery over wire P and the circuit previously traced.

In the case of the connecting circuit of a special control position battery would be found over relay B contact slc, key RA, contacts ski, and sZ2 and wire PS.

When a call distributor tests in to the assigned circuit relay B is operated and provides a holding circuit for itself at contact bl to wire P or over contact b4 to wire PS independently of key CA. Contact b2 disconnects wire N from wire EN and connects it to relays NA and NC one or both of which as the case may be being operated from battery over resistance Rl in the trunk circuit .and indicative of the characteristics of the line wire N. This circuit is completed during the operating lag of relay BA which is heavily slugged at its armature end and the relay or relays operated lock up over their left hand windings and contact b3 which also applies earth for the operation of relay BA. If the trunk line is one which, apart from the line to which it is to be connected does not require the insertion of a repeater relay NA only is operated. Contact b5 changes the circuit for relay SL by including a relay CG which operates and opens the circuit over which the release of a call from the queue is brought about as described in the col-pending application, and contact h6 lights the circuit engaged lamp CE. Relay BA operating contact bal disconnects wire N from relays NA and NC and connects: it to the answering supervisory relay AS and contact baZ lights the answering supervisory lamp ASL. Relay AS operates but the resistance is too high to allow relay SY in the trunk circuit to operate until, the speaking key being thrown, contact asl shunts relay AS to increase current ow over wire N. Contact cs2 extinguishes lamp ASL.

The operator now receives particulars of the wanted connection and proceeds to set it up in the following manner. She presses the send call key SCK which by closure of contacts not shown starts a sender coupling switch (SCS Fig. 6) hunting for the position which when found is connected to a register sender. As soon as the sender is connected a lamp is lit to inform the operator of the fact, and she then keys up the number of the wanted connection or so much of it as is to be completed automatically on the key strip common to the position, the digits keyed being transmitted by means of a code to register relays in the sender. She then presses a start key in the key strip which operates a relay to disconnect the start lead for the sender coupling switch, sends forward a starting impulse to the sender and closes contact st thereby operating relay SD. Relay SD locks up over contact sdG, its low resistance winding and contact st, and at contact sdl operates relay SE. Contacts 8d2- sd5 disconnect the speaking wires from the operators circuit and connect them over contacts sell-ce1 to the sender, contacts 8e2, se3 and se8, seS connecting wires NP and NN through to the sender likewise, thus completing four wire connections between the sender and selecting switches over which the code for marking the selectors is transmitted. Contact sel disconnects the high resistance upper winding of relay SD.

A further result of the operation of the key SCK is the connection of earth over contact sd'l and wire ST to operate relay K in the rst trunk selector (Fig. 3). Operation of relay K extends this earth back over wire P over which the relay is ultimately held. The key also closes an .a1- ternative circuit over a contact of the speaking key for lamp CE.

The code markings are now transmitted by the sender which vby the operation of a relay in the position circuit has become disconnected from the key strip and connected to the wires positive, negative, NP and NN on the outgoing side of the connecting circuit. It will be realised that if a call is to be set up by means of a reverting selector the send answer key SAK will be operated in place of the send call key SCK and the sender will become connected to the answering side of the connecting circuit.

When the connection has been set upand the speaking key is restored relays SK, SL, SD and SE release. Contact S12 falls back before contacts slcl and slcl open thus providing that the circuit over relay B is continuous during the release of relays SK and SL. Relays NB and ND are now connected to wire NP to which battery over a resistance will be connected in the trunk line circuit. Relay NB operates. If they resistance is of low value ND will alsoy operate. The operation of relay NC or ND denotes that a repeater isr required in the connection and accordingly a circuit is closed from earth over contacts rol, ncl and/or mil, nhl, mtl, h1, relay ST of the first free repeater connected to a chain start circuit to battery. Relay ST operates and connectsv latch magnet MU of the coupling switch of the seized repeater over contacts st2, hl and tl. Contact stl brings about the closure of the heating circuit for the filaments of the amplifiers, contact sts connects relay T to wiper 8 of the coupling switch the arc contact of which appropriate to the connecting circuit requiring the repeater is marked by battery potential over shunted relay RC .and contacts no2 and/or mi2, nbZ and no2. When the marked connecting circuit is found relay T operates in the above traced circuit and brings about the release of the latch magnet MU and the removal of a short circuit about relay I-I at contact tl. The same contact also connects earth over the low resistance lower winding of relay T to mark the connecting circuit busy to other coupling switches. Relay H operates in series with magnet MU but the magnet is not operated in this circuit and the relay locks up-over contact hl and at contact h2 opens the homing circuit for the coupling switch. Contact h3 substantiates contact st3, contact h4 connects the gain control relays to wiper 1, contact h5 prepares a circuit for relay I-IA, contact h6 substantiates contact stl and contact h1 disconnects the start wire from relay ST and connects it to the start wire of the next repeater connected in chain like manner to a ring start wire. Relay GA operates over wiperd'l the resistance, contact sl'l, contacts of ring call key RCN and ring answer key RAN to earth. If both relays NC and ND are operated the resistance in this circuit is short circuited and relay GB operates in addition to relay GA. Contact gal completes the circuit for relay HA which operates and at contacts haS-ha'l connects the repeater contacts hal and fm2 open'to connect balancing networks to the repeater connection. Contacts of relay GB and other contacts of relay GA are associated with the potentiometers for the control of the gain to be inserted, one or other of two fixed values of gain being inserted. When relay T operated relay RC was operated. Contact Tcl opens the start circuit, contact rc2 re-operates relay SD over its upper winding and contact SI5 to split the speaking wire circuit, contact rc3 prevents the earth over contact scZB and the lower winding of relay SD shunting out any other SD relay which it may be required to operate for the purpose of setting up another connection, contact rcB prevents operation of relay SE and contacts T04 and T05 extend connections from the repeater over wipers 3 and 4 to key MON which when thrown extends these connections to a monitoring circuit.

Supervisory relay CS is operated over wire S from earth in the outgoing trunk circuit when the connection has been extended thereto and while the speaking key is thrown contact csl shunts the relay to allow relay SY in the trunk line circuit to operate. Contact cs2 extinguishes the supervisory lamp CSL which was lit over wire SY and contact 083 starts the time checking device MR.

The operator may ring over either trunk line by pressing the appropriate key RAN, RCN. This disconnects relay GA which in turn releases relay HA to disconnect the repeater during ringing and connects battery over the positive wire to operate relay RR in the trunk line circuit, the result of which has already been described.

The operator may release either side of the connection by the depression of the appropriate key RA, RC, vand she may, as at the end of conversation, release the connection by depressing the release key RL, thereby opening the circuit over wiper P simultaneously and releasing relay T in the trunk line' circuit and relay H in the first trunk selector or reverting selector circuits, thus initiating the 'release of the connection. Relays NA and NB release also the operated relay NC, ND thereby releasing relay T. Contact tl falling back short circuits relay H which releases and releases in turn relays GA and HA. The homing circuit of the coupling switch is now closed over contacts onl, ha, h2, st2 and magnet MU and the coupling switch is horned in known manner.

If the operator wishes to recall a subscriber over an incoming trunk line, for instance after she has cleared a connection which has been subjected to delay she throws her speaking key SP and the send answer key SAK thereby operating relay RS over its winding (I) and contact sZI. Contact rsl connects earth over contact 'bl to the start wire IN to operate relay ST or GT in the reverting selector circuit, Fig. 5, to cause a iinder switch associated therewith to rotate in search of the connecting circuit which is marked in its banks by battery over the shunted relay B, contacts slcfikey RA, contacts SR1, T83, 1's2, and Wire P. When the nder switch reaches the marked contact position relay B operates, opens the starting circuit at contact bl thereby closing a locking circuit over the front of this contact and contact rsi for winding (II) of relay RS. The further results of the operation of relay B are as previously described. The key SAK being thrown in place of the key SCK the circuit for the application of the code markings to the selectors is prepared over contacts se2-se5 and the answering sideV of the connecting circuit and the inder switch of the reversing selector.

The operation of relay TC is as described in British patent No. 401,394.

It will be remembered that when the operator threw key SCK earth was extended over wire ST to operate a coupling relay K in the first trunk selector. Such a selector is shown in Fig. 3 and it will be seen to be an 8 wiper 100 outlet motor driven switch controlled by a common control circuit which includes relays for receiving the digits in code form for the setting of the switch. The control circuit is provided at the rate of one per operators position and is multipled directly to the individual trunk selectors. The trunk selector is set by one digit and outlets may be paired. that is to say a group of outlets may be utilised to serve any two digits, a signal being sent forward to the succeeding circuit to effect discrimination between the digits which can cause an outlet to be taken into use.

Since the control circuit is individual to an operators position and the operator at an incoming connecting circuit position can only deal with one call at a time it is not necessary to make any test for a selector with a free control circuit the fact that an operator is free to .deal with a call and has a free connecting circuit being suilicient to determine that the rst trunk selector and its associated control are both free. When relay K operates in the above traced circuit it locks up over contacts kl hl, wire P and key RC in the positioncircuit. Contact k2 lights the circuit engaged lamp CE in the connecting circuit over wire E and contact k3 the calling supervisory lamp CSL over wire SY. Contact 164 applies earth to the code relays AW, AX, AY, AZ in preparation for the reception of the coded digit from the sender, contact 1c5 prepares the circuit of the latch magnet LM of the switch and contacts k6, Ic'l and la other preparatory circuits. The receipt of the coded digit from the sender effects operation of vone or a pair of the code relays over the corresponding wires, positive, negative, NP, NN by the application of battery thereto. A circuit is thereupon closed from earth over contacts tl m4, one or a pair of the contacts aw, 0.126, ay, azG, contacts R5, ha3, latch magnet LM, relay RA to battery and the switch is started in searth of a free outlet to a further trunk selector or to an impulse repeater in local junction lines. The required group is marked in the contact arc of wiper B by earth over rectifier R, both windings of relay T and two or more of the contacts of the code relays according to their condition. The circuit' then continues over wiper 8, contact 7c8, wiper 6, wire P to battery on a free link, e. g. over relay GS in Fig. 1. When such is encountered high speed relay T operates and removes the short circuit from winding (I) of relay TA and the direct earth from magnet LM so that relay TA is switched into the magnet circuit. The relay operates but its resistance is too great to allow the magnet to continue operated and in consequence the latter releases and the switch is brought to rest. Contact tal closes early thereby connecting winding (II) of relay TA in a locking circuit over contact kil. Contacts ta3 and tall opening later disconnect the code relays and latch magnet respectively and contacts ta5 and tali connect earth over contact k6 to operate relays H and HA. Relay H operating opens the locking circuit of relay K, its operating circuit having been opened at contact 011.2 when the switch moved oit-normal, and extends earth over wire P to hold relays H and HA, contact h2 extending this earth to effect short circuiting of relay T. The remaining contacts of relay H and contact had switch the connection through to the switch wipers I to 5. The opening of the circuit of the code relays at Contact ta3 transmits a signal to the sender to prepare for the application of the code marking for the succeeding selector. Contacts haI and ha2 substantiate contacts k2 and k3 and contact ha3 holds open the circuit of magnet LM. Relays K and T release to be followed by relayTA whereupon the control circuit is free for further use.

If digits are paired a cross connection of winding (I) of relay PP is made to terminals shown adjacent to this winding and connected to contacts of the code relays in such manner that relay PP is operated from earth over the contacts of the code relays in response to one of the digits of a pair, this digit being the one for which a discriminating signal is to be transmitted forward. When relay PP is operated and on ther operation oi relay TA earth is applied over contact ta5, winding (II) of relay PP, contact k1, wiper I to the succeeding selector circuit wherein it brings about a discriminating action. Relay PP is held by its winding (II) until the discriminating action has been taken, relay K and consequently relay TA being held over contacts m2 and pp2.

The release of the selector takes place consequent on disconnection of earth from wire P. Relays H and HA release and the switch is horned consequent on the energisation of magnet LM over contacts hd3, R5 and rml, the last mentioned contact opening when the wipers reach their normal position to release magnet LM and stop the switch.

If, during its Search the selector is unable to find a free outlet in the marked group it rotates to a last contact position to which battery is connected in the arc of wiper 6 for the purpose oi' operating relay T. Relay T, which is connected directly to the last contact in the arc of wiper 8 and relays TA, H and HA operate as previously described and busy tone is transmitted. from apparatus connected to the last contacts in the arcs of wipers I and 2. At the same time battery is connected over wiper 4 to wire NN to bring about the release of the sender.

The relevant terminals corresponding to dead or spare lines marked by the group marking contacts of the code relays are connected to the last Contact in the first section of the arc of wiper 8, that is, to contact 5l).l If a code relevant to a spare line is received the switch is stopped in this position, the corresponding contacts in the arcs of wipers I and 2 being connected to a N. U. T. circuit, wire NN being connected to battery over wiper 4 as before.

Relay RA operates whenever the latch magnet or switch motor is energised and at contacts mi and m2 close circuits for a delayed alarm relay and for the ashing of lamp RL respectively.

If the selector has routed the call to an impulse repeater in a local junction line, battery will be connected over one of the wipers I, 2 or 3 to effect circuit changes in the sender for the setting of further switches by impulses instead of by markings.

Continuing with the description and assuming that the connection is to be set up over a second trunk selector the circuits of such a selector shown in Fig. 4 will now be described.

The second trunk selector is also an 8 wiper 100 outlet motor driven switch with a common control circuit. The switch is adapted to be set in accordance with two trains of impulses and, depending on the value of the second train to search in a marked group or to be set to an individual trunk line. vlThe switch may also search for a free alternative route if no direct route is free in groups of trunks provided with this facility and in the case of a setting to an individual line it may remain on the contacts of that line if busy until the line` becomes free. Calls which cannot otherwise be completed are directed to a storage device and when a line in the required group becomes free the stored call is released and the selector is started to research the group for the free line. Like the first trunk selector the switch is marked in accordance with a code received from the sender, the code relays being situated in the control circuit and designated AW-AZ. In the control circuit is a 25 contact ratchet driven reverse drive digit switch the magnet of which is designated DM and the wipers DMI to DMB. The digit switch is adapted to receive a signal from the first trunk selector to differentiate between pairs of digits received at the rst trunk selector in respect of which the same group of outlets is searched.

The test for a free selector involves the test for its associated control circuit and to meet this condition the test wire extends into the control circuit where battery potential indicates a free condition. This circuit extends over wire P, contacts hI, 0712 of the selector switch, 1'h2, contact I in the bank of wiper DMS of the digit switch in the control circuit, shunted relay B to battery. When the circuit is taken into use relay B is operated over this circuit in series with relay T in the first trunk selector. Contact DI closes a circuit for relay JB which operates and opens its contacts i112, and 7b3-h6 in the test-in circuits associated with the selector shown and other selectors served by the same control circuit. Contact b2 closes a circuit over contact I and wiper DMI, magnet contact dm and magnet DM which operates and interrupts its own circuit stepping the switch to its second contacts. Relay K operates in series with relay B over contact jbZand at contact lcI closes a circuit for itself against the opening of contact 0712 and at contacts k2 to la makes connections between the control circuit and the selector taken into use. Meanwhile contact jbl having closed, and assuming for the moment that no earth has been applied to wire PP in the preceding circuit relay PF operates over its right hand winding in series with magnet DM. Magnet DM will not operate in this circuit. .Contact pfl closing completes a locking circuit for the relay dependent on relay PTA and independent of magnet DM. Contact pf2 applies earth over contacts gsZ, cal, and pta3 to the code relays preparatory to the transmission by the sender of the coded digit and contacts pf3 and 'pid prepare circuits for relay PTA.

If at the time the circuit was taken into use earth was present on wire PP denoting one of a pair of digits by which the selector may have been selected magnet DM is operated over wiper DM2 contact y'hl and wire PP until the twelfth lcointact position is reached. The eiect of this been received. Relay B holds over contact :iba after the digit switch leaves its second position.

The code digit marking is now received from the sender and one or a pair of the code relays is operated applying earth to the corresponding contact in the arc of wiper DMS; at the same time the closure of contacts I of the code relays concerned completes a circuit over contacts pf or pfd, ptl, pta2 for magnet DM. The magnet operates and by self interruption of its circuit at contacts dm drives the digit switch until the contact marked over wiper DMB is reached whereupon relay PT is operated to stop further drive by opening contact ptl. This contact connects relay PTA in the circuit to earth over which the magnet was operated and the relay operates and locks up over contacts ptal and jbl. Contact ptal releases relay PF but during the slow release of this relay the code relays operated were released by the change over of Contact ptaS. Contact ptaZ disconnects the magnet circuit and contact ptad releases relay PT.

If the group of trunks designated by the received digit is to be searched the digit 1 is now transmitted by the sender and this is arranged to bring about the operation of relay AW alone. Contact aw-l closes a circuit for relay GS from earth over contacts al, ayl, acl, awl, pfS, relay GS to battery. Relay GS operates and locks up over contacts gsl and bl. Contact gs2 releases relay AW causing the register sender to prepare for the connection of the next coded digit, contact gs3 closes a circuit for the selector latch magnet LM, contact gsd closes in the test circuit of the selector, contact ys5 prepares a circuit for stopping the selector in connection with a storage circuit and contact gsl opens a testing circuit associated with spare numbers.

The digit switch is not stepped by the second digit 1 but the latch magnet LM of the selector operates in a circuit from earth over contacts tal, tbl, hc2, ca3, 983, k2, had, magnet winding relay RA to battery. The selector is rotated in search of a free trunk, designated by battery potential on the outgoing test Wire PO and ,when such a trunk is found relay TA which is of the high speed type operates in a circuit over contacts 11.6, at3, Wiper DMS, marking wire, contact arc and wiper 1, contacts kl, gsd, k6, wiper 6, wire PO to battery. Contact tal disconnects direct earth from the latch magnet circuit thereby bringing about the release of the magnet and the stopping of the switch and removes a short circuit from the right hand winding of relay HC. Relay HC now operates in series with the winding of magnet LM but the magnet itself does not operate. Relay HC locks up over contacts hcl and bl and opens thelatch magnet circuit at contact hc2. Contact hc3 closes a circuit over contact 7c3 for relays H and HA which operate and switch the connection through. Contacts Y had and h6 open further points in the circuit of magnet LM against the release of relay K. Relays H and HA hold to earth connected to wire P. The change over of contact hl releases relays K and B and these relays releasing bring Yabout the release of all the relays in the control lector switch is driven rapidly to its normal position. It being assumed that the second trunk selector has selected an outgoing trunk line earth over retard coil R in the trunk line circuit (Fig. l) brings about the release oi the sender.

If instead of l being received as the second digit one of the numbers 2-0 is received the selector does not search in the group designated by the rst digit but selects the line designated by the second digit. As one of the relays AX, AY, AZ is operated vby the second vdigit relay IN is operated in place of relay GS and locks up over contacts inl and bl, Contacts m2 and ind open in the busy signal circuits against the operation of relay CA, contact in3 completes the circuit for magnet LM, contact m5 disconnects relay HC and prepares a circuit for relay CA and contacts inlland inl close the test circuit. The selector is now rotated in search of the line which is marked over relay TA, contacts inl, relevant contacts of the code relays, marking wires, contact arc and wiper 8, contacts m6, at3, wiper DMS in the position to which it was set by the rst digit, arc and wiper 1, contacts k'l, gsd, cali to battery. When this line is reached relay TA operates in the above traced circuit stops the switch and allows relay CA to operate. Relay .CA locks up over contacts cal and bl and at contact ca disconnects the test circuit and releases relay TA and connects this relay over contact k6 and wiper 6 to the test wire to test the condition of the selected line. Contacts Ca2 and ca5 close in the busy signal circuit, contact cad releases relay IN and contact ca'l releases the code relay or relays. Contact ca3 changes over to guard magnet LM and prepare an operating circuit for relay HC. Relay IN releasing a circuit is closed for relay HC over its right hand winding, contacts m5, tbl, 11,02 and ca3. Relay HC operates and locks up over its left hand winding with the result previously described if the line is free. If the line tests busyV relay TA does not re-operate after the changeover of contact cali consequently the right hand winding of relay HC is short-circuited and this relay does not operate. Busyvtone and ilash is now transmitted back over the positive wire and Wire S respectively to give a tone to the operator and flash her supervisory lamp. The selector and control circuit are held until the line becomes free or they are released by the operator. In the former case, the re-connection of battery to the outgoing test wire when the line becomes free brings about the operation of relay TA which is followed by HC,` H, and HA as before and the connection is switched through and the control circuit is released. VThe busy signal is disconnected and the operators answering supervisory lamp glows steadily, the earth for this lamp being connected in the trunk line circuit (Fig. 1) over contact e2, relay SY and contact ysl therein, relay GS in that circuit being op'- erated over Wire PO, and Wire SO and wiper 5 in Fig. 4. Y Y

If an alternative route is available in case of congestion of the direct route the contact in the arc of wiper DMd appropriate to the group in question Vis connected to earth either directly or over a cut-off key COK which is common to all the control circuits serving `selectors having access to the group. When the digit switch has been set to contacts of a group for which an alternative route is available and relay PTA has been operated relay AT operates and at contact 

